Fate has been kind to CS Lewis in death. Where many celebrated writers assume that their reputations will live for ever, he was convinced that his books would be forgotten within five years of his passing. Experience had taught him to expect the worst – the early loss of his mother, repeated reverses in his academic career, and then the death from cancer of his wife Joy after just five years of marriage. Even news of his demise was sidelined because it coincided in November 1963 with that of the assassination of President Kennedy.

However, while many of his literary contemporaries and rivals have suffered a posthumous eclipse, Lewis's tales of Narnia, his works of Christian apologetics, and his aching reflection on grief continue to reach new generations of readers and sell more copies 50 years on from his death than they ever did when he was alive.

How and why Clive Staples Lewis – known to friends and family as Jack – has become "a cultural and literary landmark" is the question Alister McGrath sets out to answer. His book is billed as a biography, but it is simultaneously more and less than that. More in that it weaves in a thoughtful, erudite lit-crit appraisal of the writings, plus an unabashed serenade for Lewis's theology (McGrath is himself a theology professor at King's College, London and a very public defender of the faith). Less in that, though he covers key episodes familiar from other biographies, McGrath picks and chooses the details that suit his purpose of painting Lewis as a modern prophet.

Indeed he seems on occasion to lack a biographer's basic curiosity about the minutiae. So though we are told Lewis was offered the CBE in 1952, and that it was "a boost to his morale", McGrath then adds as a throwaway line that the offer was declined, without even seeming to realise that an explanation might be needed.

That, though, is a minor irritant in what is otherwise a very readable study. Indeed, McGrath may be justified in feeling that, since Lewis's life has already been so well covered by others, some of them rather better literary stylists than him, he should instead focus his energy on bringing something new to a familiar subject. The most thought-provoking sections are therefore those where he picks away at the well-known books, explains their genesis, identifies their common themes, and defines Lewis's enduring appeal.

CS Lewis was a convinced unbeliever until his late 20s. His conversion came as a result not of church attendance or reading uplifting spiritual literature, but rather extensive study of the canon of medieval literature, the bread and butter of his academic life as a don at Magdalen College, Oxford. "A young man who wishes to remain a sound atheist," Lewis wrote, "cannot be too careful of his reading. There are traps everywhere." Reading Dante, in particular, McGrath says, helped heal the trauma of active service in the first world war and guide him to God.

This book-fuelled inner journey from atheism, through theism and on to Anglicanism was, this biography suggests, akin to the snow melting in Narnia as the preface to Aslan's arrival. There was no single road-to-Damascus moment. And indeed Lewis's eventual destination, middle-of-the-road C of E rather than the more extreme end points of contemporaries such as Graham Greene and Evelyn Waugh, who pitched up in Rome, reveals much of the everyday, unsensational, everyman nature of Lewis's conversion, subsequent beliefs and therefore mass appeal.

To emphasis the absence of drama, McGrath convincingly demolishes the standard tale of Lewis taking the final step to faith during an early morning motorcycle ride with his brother to Whipsnade zoo. It was rather a gradual realisation "that there was a deeper order, grounded in the nature of God, which could be discerned – and which, once grasped, made sense of culture, history, science, and above all the acts of literary creation he valued so highly".

Lewis was not a natural evangeliser and so tried initially to dodge invitations to set down his Godly thoughts, only agreeing because he found Christian apologetics flowed much more easily from his pen than the academic papers and tomes an Oxford don is expected to produce. The Problem of Pain (1940), The Screwtape Letters (1942), Mere Christianity (1952) and A Grief Observed (1961) might then be seen as his chosen treatment for writer's block. Along with The Lion, the Witch and the Wardrobe, published in 1950, they made him famous on both sides of the Atlantic.

His reaction to being celebrated was to try to ignore it. He continued to live frugally, with his alcoholic brother and the older woman McGrath refers to as "Mrs Moore", part mother substitute, part burden and – though the question is left open – probably also his lover.

Many of his peers in academia, though, resented his "outside" success (JRR Tolkien, part of his circle of dons and the author of The Hobbit, was a rare exception). They objected instinctively to his unabashed faith but more particularly to his popularity, and so engineered a series of reverses as Lewis tried to climb the career ladder before he belatedly secured a chair at Cambridge.

McGrath makes a strong case that it is the kind of religious belief that Lewis describes – "transdenominational", non-clerical, rooted in a practical morality rather than dogma – that explains his enduring appeal. To which I would add readers' appreciation of the minor miracle that Lewis managed to find and sustain faith in the face of the tidal wave of bereavement and disappointment that engulfed his life.